It is well known that in perforating earthen formations to tap subterranean deposits such as gas or oil, the perforation is commonly accomplished by rotary well drilling tools and a drilling fluid. The drilling fluid serves to transport to the surface the material loosened by the drilling tools, such as a drill bit; to lubricate the drilling tools including the drill string thereby reducing frictional power loss; to act as a cooling fluid to absorb heat generated by the drill bit and by frictional drag of the rotating pipe; to deposit a cake or layer on the walls of the well and within the pore spaces and crevices of the wall rocks thereby sealing porous formations and reducing water loss and to seal off high pressure gas, oil or water yielding zones so that such fluids cannot enter the well bore during the drilling operation.
Both water-based and oil-based drilling fluid systems are known. The more economical water-based systems are used when practicable with oil-based systems being used where increased lubricity at the drilling head is desirable or when traversing formations which would be adversely affected by a water-based system, such as water soluble shale formations. It will be understood that while this specification refers to oil-based and water-based systems, each of these systems may contain amounts of water and oil respectively, the basing component being considered the continuous phase and generally present in an amount greater than 50% of the slurry composition. Thus, a water-based mud system would contain a greater than 50% amount of water but may also include up to 50% oil forming an oil-in-water emulsion in the mud. Similarly, an oil-based system would contain greater than 50% oil with up to 50% water being possible and forming a water-in-oil emulsion.
Oils commonly used in both oil- and water-based mud systems are commonly crude oil, diesel oil, fuel oil and the like. In U. S. Pat. No. 3,668,129, preferred oils used as lubricants in aqueous drilling fluids contain some unsaturated and aromatic hydrocarbons because they facilitate blending with other mud components. A similar preference for aromatic and unsaturated hydrocarbons is shown in U. S. Pat. Nos. 3,948,782, 3,850,248, 3,849,316 and 3,528,914, as well as Canadian Pat. No. 596,454.
More recently, due to the toxicity of aromatics present in these oils in amounts ranging over 25%, particularly in marine environments, paraffin-based mineral oils having an aromatic content of less than about 10% are being used. One commonly used mineral oil is sold by Exxon, Inc. under the trademark Mentor-28. Such mineral oils, generally comprising normal-paraffins in the C.sub. 11 to C.sub. 20 range of carbon chain length, are finding favor for use in oil-based drilling muds used in off-shore drilling operations where toxicity to marine life is an important environmental consideration.
In addition to the toxicity problems, Coast Guard and Environmental Protection Agency regulations require that there be no discharge from any marine drilling operations which causes a sheen, film or discoloration to form on the water surface. All of the above-mentioned oils would create a water surface sheen upon discharge from drilling operations. Thus, overboard discharge of drill cuttings produced during drilling operations using an oil-based mud with any of the above-mentioned oils would be prohibited without some sort of washing treatment prior to discharge. Otherwise, such cuttings would need to be accumulated and hauled to shore for treatment and/or disposal. The washing of drill cuttings requires extra equipment which raises the cost of a drilling rig while further increasing the cost of an offshore platform due to the need for additional deck space and weight capacity for such equipment. Carry off disposal is also a costly operation.
Another problem with the use of oils having a high aromatic and olefin content has been recognized by Canadian Pat. No. 649,477. In drilling so-called "wildcat" exploratory wells, it is common to use a water-based drilling mud so that cuttings and/or core sections may be inspected for UV fluorescence which indicates the presence of crude oil in the formation being traversed. Since all of the above-mentioned oils for use in drilling fluids are high in aromatics and unsaturates which would cause such fluorescence, the use of an oil-based drilling mud precludes the use of this valuable exploratory technique. In the afore-mentioned Canadian Pat. No. 649,477, the patentee provides for an oil-in-water emulsion for use as an oil-based drilling fluid which limits the aromatics and unsaturates in the oil and any emulsifier used to a level of 5% or less. The refined oil is subjected to Edeleanu (SO.sub. 2 liquid in benzene) processing followed by clay treatment to remove aromatics and unsaturates. A "thermally stable" oil which evidences no appreciable discoloration upon heating to at least 250.degree. F. is produced.